By consumption stability and by what your site can physically accommodate, not by unit price.
Bulk delivery fills a static tank from a tanker. Unit cost is lowest and handling is minimal, but it needs a permanent external installation with vehicle access, a prepared base, separation distances and usually a multi year agreement. It earns its place where consumption is high and steady.
Cylinder or portable vessel delivery on exchange costs more per litre and needs no fixed installation, and you can scale the order up or down. Most IVF laboratories and research units sit here, correctly.
The crossover is not a fixed number of litres. It arrives sooner if your vessels are efficient, because a predictable draw makes the fixed cost of an installation easier to justify. Work it out from your own measured weekly consumption rather than from sample throughput.
